Optical amplifiers in intermediate optical repeaters, low noise pre-amplifiers in receivers, and high power post-amplifiers in transmitters are important elements in most lightwave communication and transmission systems of interest. A potentially valuable embodiment of such amplifiers is the fiber amplifier which permits amplification by incorporation of dopant material such as rare earth ions in a host fiber.
At the present time, erbium is the dopant material of choice for silica-based fibers because both the pump and signal wavelengths are supported in the host fiber with relatively low intrinsic loss. Delivery of rare earth species to the reaction and deposition zone during fiber preform fabrication have been devised for standard techniques such as modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), and outside vapor deposition (OVD). These delivery methods require a high degree of accuracy and temperature control of the vapors to insure commencement of the necessary chemical reactions for incorporation of the rare earth ions. Solution doping techniques have also been proposed for incorporating low volatility rare earth ions delivered as halides into high purity fiber preforms formed by each of the techniques listed above. This doping technique also requires a certain amount of control owing to the low vapor pressure of the rare earth dopants.
In most optical amplification applications, high dopant levels on the order of several parts per million to several hundreds or thousands of parts per million are required. For distributed amplification applications, dilute dopant levels on the order of several parts per billion are desired. While the doping techniques mentioned above are capable of producing relatively uniform, high dopant level concentrations in the preform, there is no evidence to suggest that these techniques can produce the same degree of uniformity for the dilute concentrations needed in distributed amplifying fibers. For distributed amplifying fibers, it is important to provide a relatively uniform gain along the fiber length from one end to the other,
In addition to the concentration level and its uniformity of distribution in the preform, it is equally important to be able to center the dopants in the core region in the preform and fiber, to control outdiffusion of dopants from the core region of the preform and fiber, to achieve a desired concentration level regardless of magnitude, and to achieve the desired concentration level reproducibly from one preform to the next. Many of the doping techniques maintain control of the dopants to prevent outdiffusion from the core region. However, most techniques do not provide enough accuracy to center the dopant materials in the core region. Also, these doping techniques are not sufficiently controlled or controllable to achieve an a priori desired concentration level. As a result, these techniques are not well suited for a production environment which requires reproducibility from preform to preform and from fiber to fiber.